1. Field of the Invention
The present invention relates to an optical member that increases the directivity of light emitted in the form of a plane, a light source apparatus that can emit light with high directivity, a display apparatus provided with the light source apparatus, and a terminal apparatus in which the display apparatus is mounted.
2. Description of the Related Art
A liquid crystal display apparatus (hereinafter abbreviated as “LCD”) is conventionally used as a flat display apparatus. An LCD is widely used as the display apparatus of a mobile terminal. In such a mobile terminal, there are many cases in which one does not want to have other people view displayed information, and the LCD preferably has a narrow viewing angle in such cases. However, there are also cases in which information displayed on a mobile terminal is shared and viewed by other people, and the LCD preferably has a wide viewing angle in such cases. Therefore, a technology is needed for switching the viewing angle in accordance with the service conditions.
A liquid crystal display apparatus that fills such a need is disclosed in Japanese Laid-Open Patent Application No. 9-244018 (FIG. 3). FIG. 1 is a cross-sectional diagram showing the conventional liquid crystal display apparatus with a controlled viewing angle cited in Japanese Laid-Open Patent Application No. 9-244018 (FIG. 3). FIG. 2 is a perspective view showing an illumination device provided to the liquid crystal display with a controlled viewing angle. A conventional liquid crystal display apparatus 101 with a controlled viewing angle has a liquid crystal display panel 102, a scattering control device 103, and an illumination apparatus 104 in the stated order, as shown in FIG. 1.
The illumination apparatus 104 is provided with a sheet 120 that has light-blocking slits, and is also provided with an illumination unit 121, as shown in FIG. 2. The illumination unit 121 is provided with a fluorescent light tube or another light source 122, and a reflective sheet 124 for reflecting light emitted from the light source 122. The surface on the opposite side of the reflective sheet 124 is a light-emitting surface 123, as viewed from the light source 122. The light emitted from the light-emitting surface 123 is directed to the sheet 120 having light-blocking slits. In the sheet 120 having light-blocking slits, a large number of thick light-blocking materials are arranged in the form of stripes on one of the surfaces of a transparent sheet. The direction in which the light-blocking material is extended matches the perpendicular direction of the display unit. Also, the side surface of the light-blocking material is parallel to the direction that is perpendicular to the screen of the display apparatus 101.
The scattering control device 103 changes the extent to which incident light is scattered by the application of voltage. For example, a transparent state is maintained when voltage is not applied, and incident light is transmitted without being scattered. When voltage is applied, the state becomes cloudy, and incident light is scattered and transmitted.
In the conventional liquid crystal display apparatus with a controlled viewing angle, the light that is output from the light source 122 is directly emitted, or reflected by the reflective sheet 124 and emitted from the light-emitting surface 123, and is allowed to enter the sheet 120 having light-blocking slits, as shown in FIG. 2. The light is blocked by the light-blocking material when passing through the sheet 120 with light-blocking slits after having entered from a direction inclined at a fixed angle or greater in the array direction of the light-blocking material away from the direction that is perpendicular (normal direction) to the surface of the sheet 120 having light-blocking slits. For this reason, only light inclined away from the normal direction at an angle that is less than a fixed angle can pass through the sheet 120 with light-blocking slits. As a result, the directivity of light emitted from the illumination unit 121 can be increased by passing the light through a sheet 120 having light-blocking slits.
The light emitted from the sheet 120 having light-blocking slits enters the scattering control device 103. At this point, when the scattering control device 103 is in a scattering state, the incident light is scattered by the scattering control device 103 and emitted in a state of reduced directivity. Conversely, when the scattering control device 103 is in a transparent state, the incident light passes directly through the scattering control device 103 and is emitted in an unchanged state of high directivity.
The light emitted from the scattering control device 103 enters the liquid crystal display panel 102 and passes through the liquid crystal display panel 102, whereby an image is added. If the scattering control device 103 is in a scattering state at this time, light with reduced directivity will enter the liquid crystal display panel 102, pass through the liquid crystal display panel 102, and depart in all visual angle directions. The image can thereby be viewed not only from positions that correspond to the direction perpendicular to the display surface of the illumination apparatus 104 (hereinafter referred to as the “front position”), but also from positions offset from the front position in the horizontal direction of the screen (hereinafter referred to as “diagonal position”). In contrast, if the scattering control device 103 is in a transparent state, light with high directivity enters the liquid crystal display panel 102, passes through the liquid crystal display panel 102, and departs only in the front surface direction. As a result, the image can be viewed from the front position but the screen is pitch-dark when viewed from a diagonal position, and the image cannot be viewed because light that has passed through the liquid crystal display panel 102 does not arrive at the diagonal position.
Thus, in the liquid crystal display apparatus 101 with a controlled viewing angle, the scattering of light can be controlled by the scattering control device 103, and the display viewing angle can therefore also be controlled. The illumination apparatus 104 is provided with a sheet 120 having light-blocking slits, which allows the display content to be recognized only by a viewer in the front position when the scattering control device 103 is in a transparent state, because light with high directivity can be emitted toward the liquid crystal display panel 102. Therefore, the illumination apparatus 104 can be arbitrarily switched between a wide-viewing-angle display that has low viewing angle dependency and that maintains uniform display characteristics for all viewing angle directions, and a narrow-viewing-angle display that allows the display image to be viewed only from the position directly in front (front position) of the display unit.
However, the display apparatus cited in Japanese Laid-Open Patent Application No. 9-244018 (FIG. 3) has the following problems. When the display apparatus is applied to a large display screen, the perspective angle of the screen edges increases when the screen is viewed by a viewer positioned in the front position of the display apparatus. In other words, although there is a match between the line-of-view direction and the normal direction of the screen in the center area of the screen, these directions do not match at the edge portions of the screen. For this reason, when the narrow-viewing-angle display is used, light emitted from the edge portions of the screen is unlikely to reach a viewer who is positioned in the front position. As a result, an observer perceives a brightness reduction at the edge portions of the screen, and the viewability in this area is reduced. This effect is particularly marked in a display apparatus having a relatively large display screen because the perspective angle increases at the edge portions of the screen.
Disclosed in Japanese Laid-Open Patent Application No. 11-295705 (FIG. 8), for example, is a liquid crystal display apparatus in which the slit intervals of a sheet with light-blocking slits are distributed. In the liquid crystal display apparatus cited in Japanese Laid-Open Patent Application No. 11-295705 (FIG. 8) the display panel is a polymer-dispersed liquid crystal panel. FIG. 3 is a cross-sectional view showing the polymer-dispersed liquid crystal display cited in Japanese Laid-Open Patent Application No. 11-295705 (FIG. 8). The liquid crystal display apparatus 201 has a backlight unit 202, an anisotropic photoabsorption film 203, and a polymer-dispersed liquid crystal panel 204 in the stated order, as shown in FIG. 3. The backlight unit 202 is provided with a light-guide plate 205, and a light source 206 is disposed to the side of the light-guide plate 205. In the light-guide plate 205, light that has entered from the side surface of the plate is emitted from the principal surface of the plate, i.e., the surface of the side that faces the anisotropic photoabsorption film 203. Also, a photoabsorption layer 207 is disposed on the side opposite from the anisotropic photoabsorption film 203, as viewed from the light-guide plate 205.
The anisotropic photoabsorption film 203 is composed of a transparent portion 208 and an opaque portion 209, and the boundary between the two portions is perpendicular with respect to the surface of the anisotropic photoabsorption film 203. As viewed from the direction perpendicular to the surface of the anisotropic photoabsorption film 203, the transparent portion 208 is divided into a plurality of areas, the shape of each area is circular or polygonal, and the opaque portion 209 surrounds each of these areas. In the diagram, A is the maximum inside diameter of each area of the transparent portion 208, t is the thickness of the anisotropic photoabsorption film 203, and the values of the ratios (t/A) are made to be mutually different within the display area.
Two transparent substrates 210 are disposed apart from and parallel to each other in the polymer-dispersed liquid crystal panel 204, and a light-modulating layer 211 is disposed between the transparent substrates 210. The light-modulating layer 211 is composed of a polymer material 212 and a liquid crystal material 213 sealed inside the polymer material 212.
In the liquid crystal display apparatus 201, the values of the ratios (t/A) are distributed within the plane of the anisotropic photoabsorption film 203, as described above. It is believed that this technology allows the value of the ratio (t/A) at the edge portions of the screen to be kept below the value of the ratio (t/A) in the center area, whereby the brightness at the edge portions of the screen is made greater than at the center, and the brightness within the screen is kept uniform when the screed is viewed from the front position
There is a problem in this case, however, in that the directivity of the light that passes through the edge portions of the screen is lower than the directivity of the light that passes through the center area of the screen. Since the brightness of the edge portions of the screen is made relatively high, the brightness in the center area of the screen must be reduced more than necessary, and the light utilization ratio is reduced.
In view of the above situation, it is possible to consider the use of the convergent louver cited in U.S. Pat. No. 3,919,559 (FIG. 7) in place of the sheet 120 with light-blocking slits shown in FIG. 2 and the anisotropic photoabsorption film 203 shown in FIG. 3. FIG. 4 is a perspective view showing the convergent louver cited in U.S. Pat. No. 3,919,559 (FIG. 7). The conventional convergent louver 301 is in the form of a sheet, and a transparent layer 302 and light-blocking layer 303 are periodically arrayed along a parallel single direction on the surface of the convergent louver 301, as shown in FIG. 4. The boundary between the transparent layer 302 and light-blocking layer 303 is sloped toward the direction perpendicular to the surface of the convergent louver 301, the angle of the slope is low in the center area of the convergent louver 301, and the angle increases toward the edge portions along the array direction of the transparent layer 302 and light-blocking layer 303.
The direction in which light rays are restricted in each position of the screen can be oriented toward the front position by incorporating the convergent louver 301 into the display apparatus. Light in the direction traveling from the edges of the screen to the front position is thereby no longer blocked. As a result, the display apparatus has improved brightness in the edge portions of the screen as viewed from the front position in comparison with the display apparatus cited in U.S. Pat. No. 3,919,559. Since only the side surface of the light-blocking layer 303 can be seen from the front position, the effect that the light-blocking layer 303 has on the image can be reduced and viewability can be improved.
However, the above-described art has the following problems. When the convergent louver (see FIG. 4) cited in U.S. Pat. No. 3,919,559 (FIG. 7) is used as the element for restricting the direction of light rays in a display apparatus, as described above, light in the direction traveling from the edges of the screen to the front position is no longer blocked. In such a case, however, the brightness at the edge portions of the screen cannot be sufficiently improved. The reasons for this are described below.
FIG. 5 is a perspective view schematically showing the operation of a light source apparatus provided with the convergent louver cited in U.S. Pat. No. 3,919,559 (FIG. 7) as an element for restricting the direction of light rays. The light source apparatus is provided with a planar light source 421 and a convergent louver 422, as shown in FIG. 5. Since the distribution characteristics of light emitted from the planar light source 421 is uniform within the plane, it is believed that the luminous flux 401 is arrayed in an orderly manner on the light-emitting surface. In the luminous flux 401, the brightness is at its maximum in the direction perpendicular to the light-emitting surface of the planar light source 421, and this direction is the principal direction of light rays. Of the luminous flux 401, the component in the principal direction of the light rays is referred to as the principal-direction component 402, and of the luminous flux 401, the components other than the principal-direction component 402 are referred to as the lateral-direction component 403. The lateral-direction component 403 is smaller than the principal-direction component 402.
Conversely, in the convergent louver 422, a transmissive layer 410 and a light-blocking layer 411 are alternately arrayed in the same manner as the convergent louver shown in FIG. 4, and the slope angle of the boundary between the transmissive layer 410 and light-blocking layer 411 varies in this array direction.
The operation of the display apparatus is described next. The luminous flux 401, emitted from the planar light source 421 and composed of the principal-direction component 402 and lateral-direction component 403, enters the convergent louver 422. At this point, in the convergent louver 422, the lateral-direction component 403 of the luminous flux 401 that has entered the center area in the array direction of the transmissive layer 410 and light-blocking layer 411 is blocked by the light-blocking layer 411, and only the principal-direction component 402 passes through the convergent louver 422. In contrast, the principal-direction component 402 of the luminous flux 401 that has entered the two edges in the array direction in the convergent louver 422 is blocked by the light-blocking layer 411, and only a portion of the lateral-direction component 403 that is substantially parallel to the boundary between the transmissive layer 410 and light-blocking layer 411 passes through the convergent louver 422. The principal-direction component 402 of the luminous flux 401 is thereby emitted from the center area in the array direction of the convergent louver 422, and a portion of the lateral-direction component 403 is emitted from the two edge portions in the array direction.
However, since the intensity of a portion of the lateral-direction component 403 is less than the intensity of the principal-direction component 402, the two edge portions of the screen again appear dark when viewed by an observer positioned in the front position. In other words, the absorption ratio of the light varies in accordance with the slope of the light-blocking layer 411, and the transmissivity is therefore low at the edge portions in which the slope of the light-blocking layer is large, and increases near the center, where the light-blocking layer is perpendicularly arrayed. The light in the direction perpendicular to the surface of the planar light-emitting source is inefficient because more light is absorbed by the significantly sloped light-blocking layer 411. Additionally, since the photoabsorption ratio of the convergent louver 422 is distributed in the array direction, the brightness is uneven within the plane.
It is also possible to consider using as a planar light source 421 a light source in which light is isotropically emitted in order to make the transmissivity of the light uniform in the convergent louver 422. In such a case, however, the ratio of the luminous energy blocked by the light-blocking layer 411 of the convergent louver 422 increases, and the light transmission efficiency is considerably reduced. As a result, the light utilization ratio is dramatically reduced, and power consumption is increased or image quality reduced.